Re-sampling mainly involves reformatting the image data produced from the original reconstruction process in an orientation other than that which was originally produced. The process of resampling and editing is called multiplanar reconstruction or reformation (MPR). The reformatting process does not alter the MRA voxels in any way; instead it presents them in off-axis views and displays the images produced from the original reconstruction process in a new orientation.
For example MPR displays planar sections in any orientation through a 3D data volume (Fig. 2). Whereas some systems extract slices only in the planes perpendicular to the native slices i.e. sagittal, or coronal, other more sophisticated programs allow for interactive real time extraction of oblique planes. MPR should not be confused with a thick projection like subvolume MIP. Rather, it represents a single voxel section through the volume. Thus, oblique MPR can be performed but requires interpolation of image data, as in effect, it rearranges the data into a different coordinate system. For example, axially acquired MRA data can be reformatted or rearranged into sagittal MPR images.
On most systems MPR views of a 3D MRA data set are primarily obtainedin the standard orthogonal planes (i.e. axial, sagittal, and coronal views). Using a center point of interest, the operator is able to "slide" through the volume along these standard orientations. Using MPR, the operator can simultaneously visualize the arterial lumen and the adjacent structures in a localized region and interactively view features that may not be readily apparent on the native "source images" . However, the MPR requires a trained operator, who must mentally reconstruct a three-dimensional picture from two-dimensional views. Some software packages additionally permit curved reformation of image data according to an irregular orientation in space. This is particularly useful for reformations of the coronary arteries. Curved reformatting is based on two operations: interpolation and resampling, which enable signal intensities to be assigned to voxels located "somewhere between" the actual data points. It is then possible to sample a
Fig. 2.3D visualization techniques: MPR according to three orthogonal axes (left), oblique MIP (middle), surface rendered (right)
3D MRA data set along a curved plane. The technique is especially useful for visualizing stenoses in tortuous bloodvessels.
Subtraction of a precontrast data set from the arterial-phase data is a classical means of re-sampling data. This is performed routinely in x-Ray angiography (Digital Subtraction Angiography, DSA) and is now widely used in 3D CE MRA, especially for peripheral MRA of the lower extremities when subtraction of mask images is necessary to eliminate background structures and provide better visualization of smaller vessels. However, this technique is more problematic in the abdomen, where discrepanciesin breath holding between acquisitions may result in misregistration artifacts. In practice, arterial-phase CE MRA images of the abdomen are almost always diagnostic without subtraction as long asgood bolus timing has been achieved.
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